Electrooptical switch structure

ABSTRACT

An integrally formed switch structure for changing the level of light upon an electrical impedance from an elongated light source. The switch structure comprises a rotatable sleeve positioned snugly for sliding rotation around the elongated light source, the sleeve having light-opaque and light-transmissive portions. A resiliently biased detent mechanism is used for providing two fixed positions for the sleeve, one of the positions permitting light to pass from the source through the light-transmissive portion of the sleeve to the impedance and the other of the positions interposing the light-opaque portion of the sleeve between the source and the impedance to prevent light from the source from being transmitted to the impedance. The detent mechanism includes a ridge substantially parallel to the axis of rotation of the sleeve. The ridge is formed by the intersection of two surface areas on the outer surface of the sleeve so that the sleeve may be held in its operative and inoperative positions by the detent mechanism which bears against a selected one of the surface areas to retain the sleeve in one of the fixed positions. A masking projection extends from the outer surface of the sleeve adjacent the light-transmissive portion of the sleeve for optically isolating the electrical impedance from optical interference from adjoining sleeves. Finally, a lever arm is connected to the sleeve which is manually engageable to rotate the sleeve from one fixed position to the other fixed position.

United States Patent [72] Inventor Kenneth E. Fitzgerald Sudbury, Mass.

[21] Appl. No. 21,086

[22] Filed Marfl9, 1970 [45] Patented Oct. 5, 1971 [7 3] Assignee GRIComputer Corporation Newton, Mass.

[54] ELECTROOPTICAL SWITCH STRUCTURE Primary Examiner-Anthony L. BirchAttorney-Robert J. Schiller ABSTRACT: An integrally formed switchstructure for changing the level of light upon an electrical impedancefrom an elongated light source. The switch structure comprises arotatable sleeve positioned snugly for sliding rotation around theelongated light source, the sleeve having light-opaque andlight-transmissive portions. A resiliently biased detent mechanism isused for providing two fixed positions for the sleeve, one of thepositions permitting light to pass from the source through thelight-transmissive portion of the sleeve to the impedance and the otherof the positions interposing the light-opaque portion of the sleevebetween the source and the impedance to prevent light from the sourcefrom being transmitted to the impedance. The detent mechanism includes aridge substantially parallel to the axis of rotation of the sleeve. Theridge is formed by the intersection of two surface areas on the outersurface of the sleeve so that the sleeve may be held in its operativeand inoperative positions by the detent mechanism which bears against aselected one of the surface areas to retain the sleeve in one of thefixed positions. A masking projection extends from the outer surface ofthe sleeve adjacent the light-transmissive portion of the sleeve foroptically isolating the electrical impedance from optical interferencefrom adjoining sleeves. Finally, a lever arm is connected to the sleevewhich is manually engageable to rotate the sleeve from one fixedposition to the other fixed position.

ELECTROOPTICAL SWITCH STRUCTURE This invention relates to a switch andmore particularly to a novel 'electro-optical switch structureparticularly useful in multiple-switch arrays.

ln computation devices requiring a multiplicity of manually operable,two-position switches arrangedin line on a control console, the use ofmechanical switches poses several problems. The signal levels areusually quite low, hence the switches rarely carry enough current tokeep the switch contact faces clean. Additionally, mechanical switchesusually exhibit contact bounce, creating transients needing filteringand often other signal conditioning.

A principle object of the present invention is to provide an integrallyformed switch structure for performing switching electro-optically,thereby overcoming a number of the problems noted above.

The switch structure of the present invention has the advantage of beingan integrally formed unitary member which has all the necessaryfunctions provided by a single unit. The unitary switch structureprovides all the functions of (l permitting the movement between twofixed positions so as to change the level of light incident on theimpedance, (2 an integrally formed lever arm for rotating the structurebetween the two fixed positions and which arm has substantially notactile ambiguity, (3 a single sleeve integral with the arm which in onefixed position blocks light from the source to the impedance and in theother position allows light to pass from the source to the impedance, (4spacers provided integral with the sleeve for substantially preventingany mechanical interference between adjacent switch structures, and (5)'a mask for optically isolating the impedance from being interferredwith by the operation of any other switch structure adjacent thestructure affecting that impedance. Other objects of the presentinvention are to provide a switch construction particularly adapted foruse in multiple-switch systems; to provide such a switch which is low incost, simple in operation and easy to fabricate; and to provide such aswitch in a switching system particularly useful with computationalequipment employing multivibrator circuits for signal conditioning,particularly circuits employing low-level signals with fast rise andfall times such as, for example, described in copending application Ser.No. 806,903, filed on Mar. 7, 1969, entitled Rotary Toggle Switches."

To effect the foregoing and other objectives, the present inventiongenerally comprises a switch structure for changing the level of lightupon an electrical impedance from a light source, and being positivelyheld in its operative and inoperative positions by a resiliently biaseddetent mechanism, said switch comprising a rotatable sleeve positionedsnugly but for sliding rotation around saidlight source, said sleevehaving at least one light-opaque portion and one light-transmissiveportion; detent means for providing a first fixed position for saidsleeve wherein light from said source may pass through saidlight-transmissive portion to said impedance, and a second fixedposition wherein said light-opaque portion of said sleeve is interposedbetween the source and the impedance to prevent light from said sourcefrom being transmitted to said impedance, said detent means including aridge substantially parallel to the axis of rotation of said sleeve andformed by the intersection of two surface areas on the outer surface ofsaid sleeve so that said biased detent mechanism can bear against aselected one of said areas for retaining said sleeve in one 'of saidfixed positions; a masking projection extending radially outwardly in adirection substantially perpendicular to the axis of rotation from theouter surface of said sleeve adjacent the light-transmissive portion ofsaid sleeve for optically isolating the electrical impedance from anyoptical interference from any adjoining sleeve; and a level armconnected to said'sleeve, said arm being connected for rotating saidsleeve between said fixed positions wherein said sleeve isreleasablyretained by the bearing of said detent mechanism against thecorrespondino mm n5 mid surface areas.

Other objects of the present invention will in part be obvious and willin part appear hereinafter. The invention accordingly comprises theapparatus possessing the construction, combination of elements, andarrangement of parts which are exemplified in the following detaileddisclosure, and the scope of the application of of which will beindicated in the claims. I, H

For a fuller understanding of the nature and objects of the presentinvention, reference should be had to the following description anddrawings wherein: I 7

FIG. 1 is a diagram of a switching system, in which a plurality ofswitches in accordance with the present invention may be used;

FIG. 2 is a isometric view of av specific embodiment of a switchstructure of the present invention;

FIG. 3 is a'view taken along the line A-A of FIG. 2;

FIG. 4 is a view taken along the line 8-8 FIG. 2;

FIG. 5 is a view taken along the line C-C of FIG. 2; and

FIG. 6 is a view showing the effect of rotation of the switch shown inFIGS. 2-5.

Referring now to FIG. 1, there is shown an embodiment of a switchingsystem employing switches in accordance with the present invention. Thesystem includes a light source indicated generally at 20, preferably inthe form of an elongated cylinder. In the form shown, light source 20 isa light pipe, typically of a transparent synthetic polymeric plasticmaterial, such as polymethyl methacrylate or the like, having mounted atopposite ends thereof a pair of electrical lights 22 and 24. The lattertypically are small incandescent lamps mounted in the usual sockets topower leads for energizing the lamps. Alternatively, light source 20 canbe formed itself as an elongated cylindrical incandescent lamp having along central filament. In yet another form, light source 20 can be atubular electrical lamp of the fluorescent or distributed filament type.It is preferred, for reasons that will appear obvious hereafter, thatthe light source not be also a heat source; hence, to this end, thelight pipe combination with one or 'more endmounted incandescent lampsis recommended.

Mounted adjacent and in space relationship to source 20 are a pluralityof photocells 26, preferably distributed along a line parallel to thelong axis of source 20. Photocells 26 are of the type which exhibit anelectrical impedance which changes substantially in accordance with achange in the level of light incident thereon, at least exhibiting suchchange when the light level varies from substantially zero to somefinite value. Such photocells are well known in the art and need nofurther description here.

Means are provided for varying the level of light incident on each ofphotocells 26 from light source 20. Each of the photocells is connectedso that it constitutes an impedance in a positive feedback circuit in anelectrical multivibrator 38. A substantial change in the impedance ofthe photocell thus causes a change in the state of the multivibratorcircuit 38. To vary the light level, there are provided a plurality ofshuttering means, shown in FIG. 1 for simplicity in the form of rings orsleeves 28, 30 and 32. The actual form of the sleeve of the presentinvention will be described in detail with respect to FIGS. 2-6. Thesleeves of FIG. 1 are mounted so as to be rotatable normally about thelong axis of source 20. Sleeve 28, which may be made of a light-opaquematerial, contains an opening or slot 34 cut transversely across thering. The sleeve is dimensioned so that its internal diameter fitssnugly but slidingly around the circular periphery of source 20. Amanually engageable' rocker arm or handle such as tab 36 is connected tosleeve 28 as by being molded therewith and extends radially outwardlytherefrom. It will be seen that a plurality of such sleeves can bemounted coaxially about source 20 from end to end of the latter.

If tab 36 is manually engaged and sleeve 28 to be rotated, it isapparent that for some position of tab 36, slot 34 will be adjacentcorresponding photocell 26, thereby permitting light from source 20 toilluminate that photocell. In all other positions of tab 36 where theopaque body of sleeve 28 is'interposed between source 20 andcorrespondingly photocell 26, the latter is not illuminated by source85. The switching system thus described is in essence that shown inthe'aforementioned copending application Ser. No. 806,903.

FIGS. 2-5 show one embodiment of an improved switch element 50 inaccordance with the present invention. Element 50 includes a ringportion 52, which corresponds to sleeve 28 in FIG. 1, having acylindrical opening or aperture 54. Sleeve 52 is intended to be mountedfor rotation normally with the long axis of source 20 of of FIG. 1coaxial with central axis 57 of aperture 54. Sleeve 52 comprises asimple wide ring of lightopaque material, typically a synthetic plasticsuch as nylon loaded with carbon black. Sleeve 52 has a narrow openingor slot 56 which corresponds to slot 34 in FIG. 1, cut transverselyacross the ring as seen in FIGS. 2 and 4. The sleeve 52 is dimensionedso that its internal diameter across the circular cross section formedby cylindrically shaped opening 54 can fit snugly but slidinglyrotatable around the circular periphery of source 20.

A manually engageable rocker arm or lever such as bat 58 is connected tosleeve 52 as by being integrally molded therewith. Bat 58 extendssubstantially perpendicularly to the axis of rotation 57 originatingfrom a proximal end 60 closest the axis of rotation 57 to a distal end62 furthest from the axis of rotation 57. Bat 58 is tapered fromproximal end 60 to distal end 62 so that it is of greatest dimension atits distal end 62 in a circumferential direction about the axis ofrotation for reasons which will be explained in conjunction with FIG. 6.

A plurality of sleeves 52 may be mounted coaxially about source 20 fromend to end of the latter substantially without mechanical interferencewith one another when any one bat 58 is manually engaged. Sleeve 52 hasparallel side faces or edges 64 and 66 thereof each lying in a planewhich is substantially perpendicular to the axis of rotation andsubstantially parallel to one another. The shortest distance between theside faces or edges 64 and 66 is greater than the greatest dimension ofthe bat 58 in a direction parallel to the axis 57. These side faces 64and 66 act as spacers such that a plurality of sleeves 52 may be mountedfor rotation in adjacent and contacting relationship around theperiphery of source 20. This spacing prevents any mechanicalinterference between operation of adjacent bats 58 when it is desired tomanually engage any one of the bats 58 to cause rotation of itscorresponding sleeve 52. Preferably, each sleeve is dimensioned to gripsource 20 tightly enough to prevent any sliding friction betweenadjoining or contacting edges such as 64 or 66 of a pair of adjacentsleeves from causing any unwanted rotation of the sleeve not manuallyengaged.

If bat 58 is manually engaged to produce rotation of sleeve 52, it isapparent that for some position of bat 58, slot 56 will be adjacent acorresponding photocell 26, thereby permitting light from source 20 toilluminate that photocell. In all other positions of bat 58 where theopaque body of sleeve 52 is interposed between source 20 andcorrespondingly photocell 26, the latter is not illuminated by source20.

Since a plurality of sleeves 52 may be positioned in adjacent positionto one another along source 20, it is not only necessary to insure thatthere is no mechanical interference in the operation of the sleeves witheach other, but also that there is no optical crosstalk between adjacentphotocells 26. This latter problem is overcome by the provision of amasking element 72 which is an extension or projection on the end ofsleeve 52 adjacent the slot 56. Masking element 72 extends radiallyoutwardly from the sleeve 52 in a direction substantiallyperpendicularly to the axis of rotation and provides an optical baffleso that no crosstalk occurs between adjacent photocells 26. Element 72could also be fitted into a corresponding groove in a frame (not shown)to provide a mechanical guide to prevent axial motion of sleeve 52 whenthe latter is rotated.

In order to ensure proper switching of the level of light incident on aphotocell 26, it is important to provide a proper detent mechanism. Inthe embodiment of FIGS. 2-5, as shown only in FIG. 3, a detent mechanism74 is provided for releasably locking sleeve 52 in one fixed positionwherein corresponding photocell 26 is not illuminated and a second fixedposition wherein the cell is illuminated through slot 56 in sleeve 52.Detent mechanism 74 comprises ball 76, which typically may be made ofplastic, leaf spring 78 and ridge 80 on the outer periphery of oneportion of sleeve 52. Ridge 80 is formed by two angularly disposedsurface areas 81 and 83. Areas 81 and 83 diverge from ridge 80downwardly below the outer surface of sleeve 52 to from grooves 82 and84 respectively, on either side of ridge 80. Groove 82 is defined by thesurface area 8] of ridge 80 and a projecting edge 85 defined by one endof masking element 72. Groove 84 is defined by the surface area 83 ofridge 80 and an adjacent projecting edge 87 on the outer surface ofsleeve 52.

In the position shown in FIG. 3, ball 76 is resting in groove 84 and isbiased in that position against surface area 83 by spring 78. Spring 78can be a cantilever spring mounted on a frame (not shown). As bat 58 ismanually engaged and operated to rotate sleeve 52 around axis 57 in thedirection of arrow 86 in FIG. 3, ball 76 is rotated by the movement ofarea 83 under it. The ball 76 rotates without being displaced laterallyor vertically. As ball 76 rotates, ridge 80 moves under the ball suchthat when rotation stops the ball 76 will be at rest in groove 82.Spring 78 will exert a force against ball 76 so as to then bias itagainst the surface area 81. As can be seen in FIG. 3, when ball 76 isat rest in groove 84, cell 26 will be illuminated through slot 56 whichis directly opposite the cell in that position. When sleeve 52 isrotated so that ball 76 comes to rest in groove 82, the cell 26 will nolonger be illuminated since the light-opaque portion of sleeve 52, notslot 56, will be directly opposite the cell. Detent mechanism 74 is oneof any number of such designs which may be utilized to provide apositive-action, two position detent mechanism.

FIG. 6 demonstrates the importance of making bat 58 of greatestdimension at its distal end 62 furthest from the axis 57 in acircumferential direction about the axis 57. Assuming a very thin bat 58represented by line 90, when the sleeve 52 is rotated so that it ismoved from one of its positions to the other, the bat represented byline 90 will rotate through an angle of rotation of, for example, 30 toline 92. However, by making bat 58 widest at its distal end 62, rotationof sleeve 52 by operation of bat 58 about the axis of rotation 57 fromone position to the other will actually move the outer edges 94 and 96of bat 58 adjacent the end 62 thereof through a much larger angle ofrotation, which in FIG. 6 is shown to be 48. The advantage derived fromthe distal end 62 of bat 58 being wider than the rest of bat 58 is thattactile ambiguity is substantially eliminated. Tactile ambiguity can bea serious problem in two-position switches where the angle of rotationis a relatively small acute angle. It is important that merely throughthe sensory stimulation received from touching or feeling the bat orlever arm without any visual aid, the operator can determine in whichposition the switch is resting. Without this capability as provided inthe present invention by the bat 58, tactile ambiguity might well resultbetween the two positions of switch 50.

While switch 50, as shown in FIGS. 2-6, provides that throughout thelarger portion of the angle of rotation of the sleeve 52 about the lightsource 20, the correspondingly photocell 26 will remain in its darkstate, it is clear that an alternative structure can be employed inwhich the sleeve 52 is formed of a light-transparent plastic and theslot 56 is then replaced by an opaque band provided across the sleeve52. In such case, the photocell 26 would be in a state of illuminationin all angular positions of the sleeve 52 except wherein the opaque bandis interposed between the source and the photocell. In all otherrespects, this alternative switch structure is structurally andoperatively the same as that shown in FIGS. 1-6.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawing shall be interpreted in an illustrative and notin a limiting sense.

What is claimed is:

l. A switch structure for changing the level of light upon an electricalimpedance from a light source, and being positively held in itsoperative and inoperative positions by a resiliently biased detentmechanism, said switch structure comprising:

a rotatable sleeve positioned snugly but for sliding rotation aroundsaid light source, said sleeve having at least one light-opaque positionand one light-transmissive portion;

detent means for providing a first fixed position for said sleevewherein light from said source may pass through said light-transmissiveportion to said impedance, and a second fixed position wherein saidlight-opaque portion of said sleeve is interposed between the source andthe impedance to prevent light from said source from being transmittedto said impedance, said detent means including a ridge substantiallyparallel to the axis of rotation of said sleeve and formed by theintersection of two surface areas on the outer surface of said sleeve sothat said biased detent mechanism can bear against a selected one ofsaid areas for retaining said sleeve in one of said fixed positions;

a masking projection extending radially outwardly in a directionsubstantially perpendicular to the axis of rotation from the outersurface of said sleeve adjacent the light-transmissive portion of saidsleeve for optically isolating the electrical impedance from any opticalinterference from any adjoining sleeve, and

a lever arm connected to said sleeve, said arm being connected forrotating said sleeve between said fixed positions wherein said sleeve isreleasably retained by the bearing of said detent mechanism against thecorresponding one of said surface areas.

2. A switch structure as set forth in claim 1 wherein said sleeve,ridge, masking projection and lever arm are all a single, integrallyformed unitary member.

3. A switch structure as set forth in claim 1 wherein a substantialportion of said sleeve is made of light-opaque material and saidlight-transmissive portion of said sleeve is a slot in said sleeveparallel to the axis of rotation of said sleeve.

4. A switch structure as set forth in claim 1 wherein said lever arm isintegrally formed with said sleeve, said arm extending substantiallyperpendicularly to said axis of rotation and being of greatest dimensionat its distal end and in the circumferential direction about said axisto substantially eliminate any tactile ambiguity in the lever arm in thetwo fixed positions of said sleeve.

5. A switch structure as set forth in claim 1 wherein said sleeve hastwo side edges each of which lies in a plane substantially perpendicularto the axis of rotation and substantially parallel to one another, theshortest distance between said side edges being greater than thegreatest dimension of said lever arm in a direction parallel to the axisof rotation.

6. A switch structure as set forth in claim 1 wherein said surface areasforming said ridge diverge downwardly from one another below the outersurface of said sleeve.

7. A switch structure as set forth in claim 1 wherein said detentmechanism includes rotatably positioned and positive biasing meansincluding a rotatable ball adapted to be positioned against one of saidsurface areas and a spring adapted to be positioned against said balland releasably biasing said ball against said one surface area, saidball being adapted for rotation by the movement of said surface areaagainst which said ball is selectively biased when the lever arm ismanually engaged to rotate said sleeve -to said other fixed positionwhereby said ball rotates to become positively and releasably biasedagainst said other surface area.

8. A switch structure for changing the level of light upon an electricalimpedance from an elongated light source and being positive! held in itsoperative and inoperative positions by a resilient y biased detentmechanism, said switch structure comprising:

a rotatable sleeve positioned snugly but for sliding-rotation aroundsaid elongated light source, said sleeve having at least onelight-opaque portion and one light-transmissive portion, a substantialportion of said sleeve being made of light-opaque material and saidlight-transmissive portion of said sleeve being a slot in said sleeveparallel to the axis of rotation of said sleeve;

detent means for providing a first fixed position for said sleeve,wherein light from said source may pass through said light-transmissiveportion to said impedance, and a second fixed position wherein saidlight-opaque portion of said sleeve is interposed between the source andthe impedance to prevent light from said source from being transmittedto said impedance, said detent means including a ridge substantiallyparallel to the axis of rotation of said sleeve and formed by theintersection of two surface areas on the outer surface of said sleeve sothat said biased detent mechanism can bear against a selected one ofsaid areas for retaining said sleeve in one of said fixed positions;

a masking projection extending radially outwardly in a directionsubstantially perpendicularly to the axis of rotation from the outersurface of said sleeve adjacent the light-transmissive portion of saidsleeve, for electrically isolating the electrical impedance from anyoptical interference from any adjoining sleeve; and

a lever arm connected to said sleeve, said arm being connected forrotating said sleeve between said fixed positions wherein said sleeve isreleasably retained by the bearing of said mechanism against thecorresponding one of said surface areas, said lever arm being integrallyformed with said sleeve, said arm extending substantiallyperpendicularly to said axis of rotation and being of greatest dimensionat its distal end and in the circumferential direction about said axisto substantially eliminate any tactile ambiguity in the lever arm in thetwo fixed positions of said sleeve.

9. A switch structure as set forth in claim 8 wherein said surface areasforming said ridge diverge downwardly from one another below the outersurface of said sleeve.

10. A switch structure as set forth in claim 9 wherein said detentmechanism includes rotatably positioned and positive biasing meansincluding a rotatable ball adapted to be positioned against one of saidsurface areas and a spring adapted to be positioned against said balland releasably biasing said ball against said one surface area, saidball being adapted for rotation by the movement of said surface areaagainst which said ball is selectively biased when the lever arm ismanually engaged to rotate said sleeve to said other fixed positionwhereby said ball rotates to become positively and releasably biasedagainst said other surface area.

11. A switch structure as set forth in claim 10 wherein said sleeve hastwo side edges each of which lies in a plane perpendicular to the axisof rotation and parallel to one another, the shortest distance betweensaid side edges being greater than the greatest dimension of said leverarm in a direction parallel to the axis of rotation.

1. A switch structure for changing the level of light upon an electricalimpedance from a light source, and being positively held in itsoperative and inoperative positions by a resiliently biased detentmechanism, said switch structure comprising: a rotatable sleevepositioned snugly but for sliding rotation around said light source,said sleeve having at least one light-opaque position and onelight-transmissive portion; detent means for providing a first fixedposition for said sleeve wherein light from said source may pass throughsaid light-transmissive portion to said impedance, and a second fixedposition wherein said light-Opaque portion of said sleeve is interposedbetween the source and the impedance to prevent light from said sourcefrom being transmitted to said impedance, said detent means including aridge substantially parallel to the axis of rotation of said sleeve andformed by the intersection of two surface areas on the outer surface ofsaid sleeve so that said biased detent mechanism can bear against aselected one of said areas for retaining said sleeve in one of saidfixed positions; a masking projection extending radially outwardly in adirection substantially perpendicular to the axis of rotation from theouter surface of said sleeve adjacent the light-transmissive portion ofsaid sleeve for optically isolating the electrical impedance from anyoptical interference from any adjoining sleeve, and a lever armconnected to said sleeve, said arm being connected for rotating saidsleeve between said fixed positions wherein said sleeve is releasablyretained by the bearing of said detent mechanism against thecorresponding one of said surface areas.
 2. A switch structure as setforth in claim 1 wherein said sleeve, ridge, masking projection andlever arm are all a single, integrally formed unitary member.
 3. Aswitch structure as set forth in claim 1 wherein a substantial portionof said sleeve is made of light-opaque material and saidlight-transmissive portion of said sleeve is a slot in said sleeveparallel to the axis of rotation of said sleeve.
 4. A switch structureas set forth in claim 1 wherein said lever arm is integrally formed withsaid sleeve, said arm extending substantially perpendicularly to saidaxis of rotation and being of greatest dimension at its distal end andin the circumferential direction about said axis to substantiallyeliminate any tactile ambiguity in the lever arm in the two fixedpositions of said sleeve.
 5. A switch structure as set forth in claim 1wherein said sleeve has two side edges each of which lies in a planesubstantially perpendicular to the axis of rotation and substantiallyparallel to one another, the shortest distance between said side edgesbeing greater than the greatest dimension of said lever arm in adirection parallel to the axis of rotation.
 6. A switch structure as setforth in claim 1 wherein said surface areas forming said ridge divergedownwardly from one another below the outer surface of said sleeve.
 7. Aswitch structure as set forth in claim 1 wherein said detent mechanismincludes rotatably positioned and positive biasing means including arotatable ball adapted to be positioned against one of said surfaceareas and a spring adapted to be positioned against said ball andreleasably biasing said ball against said one surface area, said ballbeing adapted for rotation by the movement of said surface area againstwhich said ball is selectively biased when the lever arm is manuallyengaged to rotate said sleeve to said other fixed position whereby saidball rotates to become positively and releasably biased against saidother surface area.
 8. A switch structure for changing the level oflight upon an electrical impedance from an elongated light source andbeing positively held in its operative and inoperative positions by aresiliently biased detent mechanism, said switch structure comprising: arotatable sleeve positioned snugly but for sliding rotation around saidelongated light source, said sleeve having at least one light-opaqueportion and one light-transmissive portion, a substantial portion ofsaid sleeve being made of light-opaque material and saidlight-transmissive portion of said sleeve being a slot in said sleeveparallel to the axis of rotation of said sleeve; detent means forproviding a first fixed position for said sleeve, wherein light fromsaid source may pass through said light-transmissive portion to saidimpedance, and a second fixed position wherein said light-opaque portionof said sleeve is interposed between the source and the impedance toprevent light from said source from being transmitted to said impedance,said detent means including a ridge substantially parallel to the axisof rotation of said sleeve and formed by the intersection of two surfaceareas on the outer surface of said sleeve so that said biased detentmechanism can bear against a selected one of said areas for retainingsaid sleeve in one of said fixed positions; a masking projectionextending radially outwardly in a direction substantiallyperpendicularly to the axis of rotation from the outer surface of saidsleeve adjacent the light-transmissive portion of said sleeve, forelectrically isolating the electrical impedance from any opticalinterference from any adjoining sleeve; and a lever arm connected tosaid sleeve, said arm being connected for rotating said sleeve betweensaid fixed positions wherein said sleeve is releasably retained by thebearing of said mechanism against the corresponding one of said surfaceareas, said lever arm being integrally formed with said sleeve, said armextending substantially perpendicularly to said axis of rotation andbeing of greatest dimension at its distal end and in the circumferentialdirection about said axis to substantially eliminate any tactileambiguity in the lever arm in the two fixed positions of said sleeve. 9.A switch structure as set forth in claim 8 wherein said surface areasforming said ridge diverge downwardly from one another below the outersurface of said sleeve.
 10. A switch structure as set forth in claim 9wherein said detent mechanism includes rotatably positioned and positivebiasing means including a rotatable ball adapted to be positionedagainst one of said surface areas and a spring adapted to be positionedagainst said ball and releasably biasing said ball against said onesurface area, said ball being adapted for rotation by the movement ofsaid surface area against which said ball is selectively biased when thelever arm is manually engaged to rotate said sleeve to said other fixedposition whereby said ball rotates to become positively and releasablybiased against said other surface area.
 11. A switch structure as setforth in claim 10 wherein said sleeve has two side edges each of whichlies in a plane perpendicular to the axis of rotation and parallel toone another, the shortest distance between said side edges being greaterthan the greatest dimension of said lever arm in a direction parallel tothe axis of rotation.